The present invention relates to measurement of the standard visual range, employing a measuring signal and a reference signal, and to a circuit arrangement therefor.
The visual range of any object which is disposed in front of or above the horizon is generally understood to be the distance up to which an observer with normal eyesight can still discern it. The value of such distance depends mainly on the clearness of the air, the ambient light conditions, and certain characteristics of the object and its environment and of the normal eye. The standard visual range, which is an accurate and long established indication of the visibility distance in the atmosphere, is physically clearly defined by the following formula: ##EQU1## WHERE V.sub.N is the standard range and z is the atmospheric extinction coefficient. The units in which V.sub.N and z are expressed in the equation are km and km.sup.-1, respectively 3.91 is a constant. The equation describes Koschmieder's theory.
Experience has shown that a beam of light is attenuated in the atmosphere mainly by the fact that the molecules of the air and the particles of the aerosol are scatter part of the light from the beam in all directions. Absorption is generally negligible and reaches the order of magnitude of scattering only in dense smoke or the like.
In practice the following is a good approximation of the standard visual range: ##EQU2## WHERE B IS THE COEFFICIENT OF ATMOSPHERIC SCATTERING. The relation between the scattering coefficient which is desired to be measured and the scattered light which is measured is given by the scattering function .beta.' (.phi.) which indicates the light intensity of the light scattered in the direction .phi. relative to its primary radiation direction. The scattering coefficient is then: ##EQU3##
It is known to determine the standard visual range, V.sub.N, with the aid of transmissiometers or scattered light recorders. Generally a good visibility measuring instrument processes reference signals and measured signals and the standard visual range results from a multiplication of a calibrating factor K with the quotient of such a reference signal and measured signal.
Thus a scattered light recorder is known which is provided with a pulsed lamp which is triggered periodically and then produces an amount of scattered light in the scattering volume which amount is greater as the standard visual range is smaller. An optical device equipped with lenses collects part of this scattered light through a wedge filter on a photocell, to produce the measured signal. A small proportion of the primary pulsed light impinges on a second photocell, as comparison light, to produce the reference signal. This proportion can be varied with a compensating aperture and the scale of the instrument can be shifted with the aid of a standard calibration so that the measured standard visual ranges are indicated correctly.
The difference between the current pulses generated by the pulsed light in the photocells is amplified in an amplifier with high feedback and is then expanded in two expansion stages and integrated over a plurality of pulses. If the sum of the positive and negative polarity portions of the signal exceeds a given threshold value, which corresponds, for example, to a deviation of .+-.5% from the standard visual range being indicated, an electrical gate is enabled for the corresponding control current of a servo motor. The servo motor then regulates the wedge filter so that the pulse difference at the input is reduced to an amount which corresponds to a threshold value &lt;.+-.5%. The position of the calibrated wedge filter, which is transmitted by a remote sensor to a recording instrument, is then a measure for the standard visual range. It is indicated on a logarithmic scale.
This scattered light recorder, when used with a servo system, exhibits an unavoidable and often undesirable time constant in the indication. The analog output also permits transmission of the measured values or transfer to digital process controls only after complicated analog-digital conversion of the measured value.